You need direct access to the database with something like phpMyAdmin. If the posts are still in the database, you will be able to get them back. If they’re not there, you might have access to something like binary logs for MySQL, but that’s a long shot.

Unfortunately, not only they aren’t in the DB, but I don’t have a usable backup either (the last full backup I have is from about two weeks ago and the daily backup has the same thing as.. what i see).

Another weird thing is that some days ago, in my Akismet spam list I found some posts from way back (that weren’t spam, but.. why check them if they were already “accepted” ?).

So I suspect that something might have triggered a full check and it automagically deleted what it considered spam or something..